Multiple sclerosis (MS), the most common inflammatory disease of the central nervous system (CNS), affects more than 2.5 million people worldwide. MS is characterized by perivascular inflammation in the CNS, demyelination of nerve fibers as well as axonal damage. The resulting interruption of motor and sensory impulses as they pass through demyelinated regions of the brain or spinal cord often leads to visual disturbances, incontinence as well as sensory and motor disturbances. The prevalence of this disease is estimated to be at least three times greater in females than males and while genetic, hormonal and immune response differences have been implicated, the basis for this gender bias is still not fully understood. MS is considered to be autoimmune in nature and myelin-specific CD4+ Th1 and Th17 cells are major orchestrators of the CNS inflammation. It is assumed that these cells are initially activated in peripheral lymphoid organs but must cross the relatively impermeable blood-brain barrier (BBB) to become reactivated in the CNS. Other innate immune cells that infiltrate or reside in the CNS contribute to inflammation-mediated neurological damage. Many of these events have been initially defined by studying the similar, albeit imperfect rodent model of MS, experimental allergic/autoimmune encephalomyelitis (EAE). Mast cells are granulocytes that reside in most tissues and are among the innate cells that exert an important amplifying effect on disease severity in females. Mice bearing mutations in Kit, which encodes the SCF receptor, ckit, fail to develop mast cells and have been used to study the role of mast cells in a relapsing-remitting EAE model. We previously demonstrated that female SJLW/Wv mice exhibit attenuated EAE, a phenotype dependent on mast cells. However, male SJLW/Wv mice have exacerbated disease, indicating either mast cells or other c-kit related defects are pathologic in males. This is not due to disparate serum testosterone levels between wild type and ckitW/Wv males, which show no significant differences. There are at least two explanations for these observations: i) Male mast cells, under the influence of sex hormones such as testosterone, show distinct responses in the context of disease compared to female mast cells; and/or ii) SCF, the ligand for c-kit, exerts a neuroprotective effect in concert with testosterone that acts to minimize immune-mediated damage in the CNS. The specific aims of this study are: 1) Compare the events (e.g. T cell priming in the periphery, inflammatory cell entry to the CNS, local CNS inflammatory responses) that lead to development of EAE in wild type versus ckitW/Wv male mice to determine where c-kit signals exert their protective influence 2) Identify whether and how mast cells or other c-kit related factors alter disease susceptibility in males. PUBLIC HEALTH RELEVANCE: It is still not understood why males are generally less susceptible to autoimmune disease than females. We observed that mutations in the Kit gene (c-kitW/Wv), the receptor for stem cell factor, protect females from developing severe disease in a rodent model of multiple sclerosis, but exacerbate disease in males. These mice provide a perfect system to explore the how c-kit signaling, in concert with influences from male sex hormones, confers neuroprotection and may lead to better therapies for this devastating CNS disease.